This invention relates to a process for the production of filled molding powders of polymers, particularly to such a process wherein the powder includes polybenzimidazole ("PBI") or another thermally intractable polymer.
Polymers are used to make a multitude of products, including a wide variety of molded articles. These molded articles often must have specific attributes that may not be provided by unfilled polymers. In such cases, it may be desirable to use polymers filled with materials such as graphite powder, chopped glass, mica flakes, carbon fibers, glass fibers, metallic powders, or other fillers. The filler may advantageously alter the mechanical or thermal properties of the polymer. Filled polymers also may be used to reduce costs, or for other reasons.
A variety of filled polymers are known in the art. For example, polyesters may be filled with talc or hollow glass spheres to improve thermal or mechanical properties, thermoplastics may be filled with metallic powders to produce thermally or electrically conductive articles, and ground silica may be used in polyurethanes to form potting compounds having favorable dielectric properties. See: Encyclopedia of Polymer Science and Engineering, Mark, Bikales, Overberger, & Menges, Eds., 2d Ed., Vol. 7, pp. 53-73, John Wiley & Sons, Inc., 1987.
Generally, the polymer to be filled is one that is easily melt processible so that it will readily flow around the filler particles during molding to produce an article having a relatively uniform composition. The ability of such polymers to completely surround the filler material minimizes potentially detrimental voids and heterogenous areas within the article.
Thermally intractable or infusible polymers such as PBI are not amenable to melt processing with filler materials. Although these polymers generally may be hot-pressed or cold-pressed and sintered by compression molding techniques, it is difficult to use these polymers to make filled molded articles because the polymer does not melt and flow around the filler particles.
U.S. Pat. No. 2,639,278 describes a method for producing finely divided nylon particles by precipitating the polymer from solution. U.S. Pat. No. 2,695,425 describes a method for making sintered nylon products using a powder made according to the teaching of the '278 patent.